Electrical cables are essential for the transmission of information and power. Because electrical cables have widely differing performance requirements and must function in various adverse environments, a large amount of technology has been developed in the field of electrical cables.
In particular, in the field of telecommunications cables, which usually contain multiple insulate conductors, it is a common practice to surround the insulated conductors with one or more protective sheaths. Typically the outer sheath is seamless plastic composition which provides water proofing and electrical insulation. Immediately underlying this plastic sheath is a metal sheath which provides strength and resistance to external damage.
Some cable geometries employ a metal sheath which is not bonded at the overlapping seam or is only weakly bonded. Typically, the metal shield in such cases is aluminum. When a steel shield is used it may be bonded to the outer plastic sheath by using a steel sheath tape preform coated with a plastic material which has adhesion properties where contacted with the extruded plastic of the outer sheath. This form of cable is prone to "zippering" which occurs when the cable is sharply bent or twisted, there is relative movement in the metal sheath overlap and and the bonded metal sheath edge at the overlap cuts the plastic sheath penetrating it and/or the outer plastic sheath elongates to failure along the metal sheath seam.
FIG. 1 illustrates a typical prior art geometrical arrangement of metallic and plastic sheathing. In FIG. 1 the underlying metallic shield portion of the sheath 10 overlaps as shown in area A. The metal shield is bonded to itself at the area of overlap A and in the prior art patent from which FIG. 1 is taken (U.S. Pat. No. 4,477,298) the metallic shield is described as having an organic coating, applied to the metal strip from which the sheath is formed, which permits bonding between the metal and plastic sheaths.
As shown in FIG. 1 the adhesive 15 which bonds the metal sheath edges 11, 12 together, is placed between two essentially parallel portions of the metal sheath edges 11, 12 with no edge constraint on adhesive flow. This allows lateral flow of adhesive 15 which can flow both inwardly and outwardly of the seam overlap causing several problems. Avoidance of excessive lateral adhesive flow requires close control of the volume of adhesive applied and the correlation between the volume of adhesive applied and the distance between the sheath portions to be joined. The sheath joint configuration shown in FIG. 1 is produced by a forming method wherein a corrugated sheath preform is formed from a flat strip to a sheath without preliminary edge treatment.
The art has also appreciated that a protruding metal sheath edge can cut the external plastic sheath and has proposed bending the overlapping metal sheath edge so that it is less prone to cut the plastic sheath (U.S. Pat. No. 4,404,720). In general, edge forming as practiced by the art has not included edge forming of the metal sheath preform as a separate step prior to forming the preform about the cable and has not included preforming both edges of the sheath preform so that they cooperate to form an adhesive retaining cavity in the end product.